Control systems often are required to work under extreme environmental conditions including high temperature and pressure. One aspect to the proper control involves the operation of position sensors under such conditions. The control system needs accurate position information from sensors in order to properly control machine movements in the system. An example environment where such conditions exist, involves the position measurement and control of control rods that are raised and lowered in a nuclear reactor. The nuclear reactor initiates and controls a sustained nuclear chain reaction via the raising and lowering of the control rods in the reactor core which ultimately requires controlled, precise, and reliable positioning to meet the ardent design constraints imposed on the reactor. Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships, for example. Heat from nuclear fission is passed to a working fluid (water or gas), which runs through turbines.
In some modern reactors, where the reactor internal components are housed in a single pressure vessel, measurement of the position is not possible using means previously utilized on commercial nuclear reactors. These reactor designs require sensors that must operate in an environment with temperatures over 300° C. and pressures over 2000 PSI as well as moderate vibration and radiation. Other design constraints require that the sensors provide a high resolution measurement of position (e.g., within 50 mils) over a stroke of about 100 inches and also have a design life of at least 60 years.